\begin{verbatim}
ok(X,Y) :-     ((Y - 500) / X) < 10, 
               ((Y  + 500)/ X) > 10.
run(X,Y) :- (ok(X,Y) -> print(user_output, 'true.\n'); 
            print(user_output, 'fail.\n')).
\end{verbatim}
